The piston diaphragm pump known from German Pat. No. 2 843 054 has proved to be extremely advantageous. When there is an underpressure in the piston work chamber, the refill valve cannot open until the diaphragm, which is deflected toward the piston work chamber during the suction stroke, displaces the support plate counter to the spring force and thus allows the refill valve to open.
In this design, however, a central protrusion is firmly connected to the support plate, which results in a number of serious disadvantages. The apparatus is vulnerable to canting and soiling, which may be caused by material wearing off from a gasket. As the finished size of the pump increases, there is also an increase in the masses which must be accelerated, further impairing pump operation. The waste space is relatively large and makes ventilation difficult. Finally, because of the large cross section of the protrusion, increased friction losses occur, so that the internal pressure losses are correspondingly high as well.